Some electrical and electronic circuits generate undesirable electromagnetic emissions and some circuits are adversely affected by electromagnetic emissions from other circuits. There are various causes for electromagnetic emission and for sensitivity to emissions. Current levels, conductor lengths, inductance and high frequency operation, and high slew rates (e.g., from switching inductive loads) contribute to generating electromagnetic interference or “EMI.” High amplifier gains, low common mode rejection, low signal to noise ratios and other factors contribute to circuit sensitivity to incoming noise.
One technique for blocking the propagation of emissions, either incoming or outgoing, is to place a conductive barrier across the propagation path. An incident electromagnetic signal produces current in the conductive barrier, but the propagating signal is attenuated, particularly if the barrier is connected along a low resistance path to the applicable signal ground reference.
Therefore, problematic sensitive and/or high frequency circuits are often shielded by grounded conductive barriers. In one example, box-like sheet metal structures wholly or partly enclose around such circuits. In other examples, enclosures are formed of conductive polymers, or coated with conductive foils or conductive paints. The enclosures can be rigid or flexible, and can be box shaped, cylindrical, domed or otherwise shaped to define an enclosure of conductive material around the applicable circuit. It is also possible to laminate over circuit elements with a relatively close fitting conductive coating or flexible cover. However, there may be cooling interests as well as shielding interests. A conductive enclosure around a circuit can have holes or slots, or can be constructed using a conductive mesh screen. Shields having holes or slots or other openings can be effective as conductive barriers for frequencies at and below a characteristic frequency related to the size of the holes. For relatively higher frequency shielding, any holes through the shield barrier need to be relatively smaller, etc.
In connection with a printed circuit board arrangement, a rectilinear sheet metal box often is used as all or part of the shield barrier. A box can be formed by folding an integral sheet and/or attaching together two or more integral sheets so that the sheets together form a conductive barrier in the required shape of a box or cylinder or tube, etc.
Assuming the example of a rectilinear box on a printed circuit board, conductive portions of the board can define part of the barrier around a given circuit element, or barriers can be provided on opposite sides if necessary. On a given side, standing conductive walls of thin sheet metal can extend from the plane of the circuit board, e.g., extending perpendicularly upwardly from a folded flange attached to the board, or carried by one or more integral tabs that engage openings on or through the board as attachment pegs, feet, floor panel elements or the like.
The standing wall elements act as the panels of a fence defining a perimeter and keeping the EMI emissions of the circuit in or out. The panels need to be mechanically mounted to remain in position, and electrically connected to one another and to one or more points on the circuit board, typically a common ground point. Electrical and mechanical connections can be made between adjacent conductive walls. Two adjacent panels can be attached to an intervening post that provides a secure attachment to the circuit card. A full enclosure also requires a cover, e.g., parallel to and spaced from the circuit board, and also connected electrically to the walls and thereby to the circuit ground or other point of reference.
The sheet metal elements of shield enclosures are typically inexpensive thin sheet metal stampings of aluminum, stainless steel or another material. There may be instances where it is desirable, e.g., for compactness and to reduce internal enclosure dimensions, to provide a shield with a complicated shape. This could necessitate multiple metal forming steps, such as successive folds made in a sheet metal stamping operation. More often it is desirable to minimize expense by using a minimum of stamping steps, to produce a simple structure.
An exemplary simple structure could resemble the walls and lid of a shoe box, with rectilinear standing walls having a rectangular footprint, covered by a lid having downwardly turned flange edges. Some structure is needed to ensure that the lid remains fixed on the side walls. This could be accomplished by providing a snug frictional fit, but a snug fit can be demanding of precision in the shape and dimensions of the inter-fitting parts, leading to expense. Alternatively, one of the lid or walls might be provided with slots, for receiving tabs associated the other of the lid or walls. The tabs can be barbed or arranged to be passed through a slot and then twisted or bent over to form a lock. These assembly steps such as fitting and bending over tabs also generate some expense.
It would be desirable to provide a technique whereby simple shielding enclosures could be made without the need for great precision in the size and shaping of the parts, so as to minimize expense. At the same time, however, such shielding enclosures need to be mechanically tight and secure as well as providing dependable low resistance electrical connections.
Electrical connections with conductors that are enclosed by insulation in cables can be made by providing a so-called insulation displacement connector or “IDC” structure. There are several alternative IDC structures in use. The typical connector includes an insulation-piercing or slicing part that is electrically conductive, and is intended to penetrate the insulation that encloses the cable conductor(s). Additionally, some mechanical arrangements are needed to position and support the cable while applying pressure between the penetrating part and the cable, so as to cut through the insulation to the conductor. The connector is termed an insulation displacement connector because when cutting through the insulation, the conductive penetrating part displaces the insulation along its path.
One type of insulation displacement connector comprises a conductive plate portion having a vee-shaped groove opening at one edge. A wire or similar conductor with insulation thereon is forced by a mechanical clamping arrangement toward the bottom of the vee-shaped groove. As the insulated wire is forced toward the bottom of the vee-groove, the converging walls of the groove function as knife edges to displace the insulation. At some extent of progress toward the bottom of the vee-shaped groove, one of the knife edges comes into contact with the wire, and when the lateral space between the converging walls of the groove is equal to the diameter of the wire, the wire comes into contact with both of the converging walls.
It is not desirable to shear off the conductor in a guillotine fashion, which might occur from applying sufficient pressure on the wire when disposed at the apex or bottom of the vee-groove that the wire is cut through. On the other hand, it is necessary to hold the conductive wire securely in contact against the conductive edges of the groove to maintain electrical contact.
In order to reduce the need for precise positioning or limitation on the pressure applied, the space between the converging knife edge walls of the vee-groove can be extended at the bottom of the vee groove by a slot along the center line of the vee groove. The slot has parallel edges that each join to one of the converging edges leading into the bottom of the groove. The spacing between the parallel edges is just slightly smaller than the outside diameter of the wire. In this way, pressing the insulated wire toward the bottom of the vee groove causes the converging knife edges to slice through the insulation. When the wire arrives near the bottom of the vee groove, the insulation on both opposite sides of the wire has been displaced. By further forcing the wire downwardly into the slot, there is some deformation or abrasion of the wire surface, but the wire is mechanically and electrically engaged in the IDC connection.
Connection of the insulated wire and its engagement in the vee groove generally require that the longitudinal axis of the wire pass at least somewhat perpendicularly through a plane defined by the knife edges. It is possible to vary the angle from perpendicular, but only up to a point. As the angle of incidence of the wire relative to the plane of the knife edges becomes more and more acute, the tendency increases to shear off the wire. If the wire is parallel to the plane, the wire cannot engage between the knife edges.
A bare wire could be engaged in a slot that is dimensioned to engage opposite sides of the wire between parallel edges. If there is no insulation to be displaced, one might dispense with having a vee groove and converging knife edges. The wire could be moved laterally to find the entry to the slot and pressed down into a slot defined between parallel walls. Assuming that the slot width and the wire outside diameter are approximately equal, or assuming that the wire diameter is only slightly larger than the slot at rest, the result would be mechanical engagement and electrical contact.
It would be advantageous if adjacent panels that reside in parallel planes (such as a lid flange overlapping the top edge of a box wall) could be mechanically and electrically attached as easily as engaging a wire in the vee groove of an insulation displacement connector. It would also be advantageous if panels that are at right angles and meet along an edge could be mechanically and electrically attached so easily. However, such panels are not arranged so that their edges cross perpendicularly. If arranged to cross perpendicularly (for example if a depending lid flange is lengthened to protrude beyond a standing side or end wall of a box), there is a resulting gap that may detract from shielding performance.